A secondary cell such as a lithium ion secondary cell has a small size and a large capacity, and thus has been used in a wide range of fields such as a cellular phone and a notebook PC. The performance of the lithium ion secondary cell is determined by materials of a positive electrode, a negative electrode, and an electrolyte, which constitute the secondary cell. Among these, research and development of an active material that is included in an electrode has been actively performed. Currently, a carbon-based material such as graphite may be exemplified as a negative-electrode active material that is generally used. A carbon negative electrode using graphite and the like as the negative-electrode active material has an intercalation reaction. Therefore, the carbon active material has satisfactory cycle characteristics, but high capacity is difficult to achieve. Accordingly, a silicon-based material such as silicon and silicon oxide, which has capacity higher than that of carbon, has been examined.
The silicon-based material has high capacity, 1000 mAh/g or more, by alloying with lithium. However, when the silicon-based material such as silicon and silicon oxide is used as the negative-electrode active material, it is known that the negative-electrode active material is expanded and contracted due to a charge and discharge cycle, and thus the volume of the negative-electrode active material varies. When the negative-electrode active material is expanded or contracted, the following problems occur. A load is applied to a binding agent that plays a role of maintaining the negative-electrode active material to a current collector, and thus adhesiveness between the negative-electrode active material and the current collector decreases, or a conduction path inside the electrode is broken, and as a result, capacity significantly decreases. In addition, a stress occurs in the negative-electrode active material due to repetition of expansion and contraction, and the negative-electrode active material becomes fine, and thus the negative-electrode active material is detached from the electrode. Due to these various kinds of problems, there is a problem in that the cycle characteristics become poor.
Therefore, as the silicon-based material, use of silicon oxides (SiOx: x approximately satisfies an expression 0.5×1.5) has been examined. It is known that when being heat-treated, SiO is discomposed into Si and SiO2. This is called a disproportional reaction, and in a case of uniform solid silicon monoxide SiO in which a ratio of Si and O is approximately 1:1, the silicon monoxide is separated into two phases including a Si phase and a SiO2 phase due to an internal reaction of a solid. The Si phase that is obtained by the separation is very fine. In addition, the SiO2 phase that covers the Si phase has a function of suppressing decomposition of an electrolytic solution. Accordingly, a problem of volume variation remains still, but a secondary cell using the negative-electrode active material composed of the SiOx decomposed into the Si phase and the SiO2 phase is excellent in cycle characteristics.
In addition, as the negative-electrode active material, use of iron oxide, which has low toxicity and is abundant in resources and is not expensive, has been suggested. With regard to the iron oxide (Fe2O3), a reaction progresses to a transition region due to intercalation of Li. At this time, a theoretic capacity is 1007 mAh/g, but at the first cycle, actual charge and discharge efficiency is 70%, and thus sufficient electrode performance may not be exhibited. In addition, in the iron oxide, there is also a problem in that a reaction speed of a cell is slow.
An electrode that uses the negative-electrode active materials in combination is disclosed in Patent Literature 1. Patent Literature 1 discloses an electrode provided with a current collector and a metal oxide-containing layer provided to the current collector. The metal oxide-containing layer contains metal oxide particles including a metal oxide and SiOx (0≦x≦2).
In Patent Literature 1, the metal oxide particles and SiOx are mixed in, excellent cycle characteristics are obtained. As a reason for obtaining the effect, Patent Literature 1 discloses the following reasons. When metal oxide particles are included, a stress that occurs due to volume expansion of SiOx, which accompanies intercalation and deintercalation of lithium ions, is mitigated, and thus deformation of the metal oxide-containing layer or peeling from the current collector is suppressed, whereby excellent cycle characteristics are obtained. In addition, Patent Literature 1 discloses that not only the SiOx but also the metal oxide contribute to the intercalation and deintercalation of the lithium ions, and thus electric capacity may be improved.